High temperature battery cells--most notably sodium/sulfur battery cells--in which the electrolyte/separator takes the form of a plurality of cation-permeable hollow fibers or tubules, are now well known. (See, for example, U.S. Pat. Nos. 3,476,602; 3,765,944 and 3,791,868.) In a typical such cell, the electrolyte/separator takes the form of a plurality of generally parallel, Na.sup.+ ion-permeable tubules or hollow fiber lengths, each open at one end and closed at the other end. The open ends terminate in the upper surface of a horizontal, disc-like "tubesheet" or wall member, through which the fibers pass in sealing engagement and from the lower surface of which the fibers depend as closed-ended portions.
The tubesheet and fibers constitute a (sub-) assembly which in turn is usually formed as part of a more complex assembly including a carbon-coated aluminum foil disposed below the tubesheet as a spiralled wrap between successive, generally concentric rows of the fibers and extending as a skirt hanging below the closed fiber ends. The space between the successive layers of this skirt is occupied by a relatively thick, aluminum spacer tape. The assembly may also include a central, vertically oriented, aluminum mandrel (tube or rod) extending below the surface formed by the adjacent edges of the skirt and tape, plus several heli-arc weld beads extending radially across the latter surface to the mandrel. (The foil, tape and mandrel function in the finished cell as a current collecting/distributing means.)
An assembly of the latter type may be formed by rolling up (on the mandrel) a wide ribbon of the foil on which the spacer tape and the fiber lengths are appropriately disposed. A bead of a viscous paste of powdered tubesheet glass and a liquid vehicle is extruded at the nip of the developing roll, adjacent to the open fiber ends. The successive, adjacent "wraps" of this bead merge to form a coherent disc which is "green cured" by heating the assembly in vacuo to remove the liquid vehicle. The assembly is then further heated to a temperature, below the distortion temperature of the fibers, at which the glass powder particles fuse together and bond to the fibers.
It is not easy to devise or find a specific tubesheet material which is suitable for use with a given specific fiber material under a particular set of conditions. To be suitable for such use, the candidate material must meet each of some seven different requirements. In a sodium/sulfur battery cell, for instance, the tubesheet material must be inert to (not detrimentally reactive to an intolerable extent with) both sodium and sulfur/sodium polysulfide mixtures, at temperatures of up to at least 325.degree. C. It must also be electronically non-conducting. Further, the candidate material must be reducible to a finely ground powder which, in turn, can be slurried with a readily evaporated or decomposed liquid vehicle to form a viscous, yet extrudeable, paste. In addition, the powder residue left after the vehicle has been removed must have enough coherence to maintain the shape that was imposed on the slurry. The powder must also be fusible, to form a rigid, fluid-tight wall member sealingly bonded to the fibers, at a temperature such that the fibers do not become appreciably distorted in the length of time required to effect the fusion. Finally, the thermal coefficients of expansion of the fiber and tubesheet materials must substantially match, at least over the range of temperatures at which the cells are made, stored and used.
It has now been recognized that still another requirement must be met, if breakage of fibers in tubesheet/fiber assemblies is to be minimized. That is, the tubesheet material should not "flux" the fibers in such manner or to such an extent that they are markedly weakened where they emerge from the tubesheet.
The most satisfactory combination of fiber and tubesheet materials previously found for use in high temperature (Na/S) battery cells was that in which the fibers have compositions essentially as disclosed in U.S. Pat. Nos. 3,829,331 and 4,050,915 and the tubesheet composition is essentially that disclosed in U.S. Pat. No. 3,197,490. Typical such fiber and tubesheet compositions are:
Fibers: Na.sub.2 O, 2B.sub.2 O.sub.3, 0.16NaCl, 0.2 SiO.sub.2 PA1 Tubesheet: Na.sub.2 O, 13.88 B.sub.2 O.sub.3. PA1 a tubesheet/hollow fiber assembly comprising a wall member with which a plurality of hollow fiber lengths passing therethrough are engaged, PA1 said wall member consisting essentially of a glass having the composition PA1 Na.sub.2 O, from about 1 to about 6.1 mole %; PA1 SiO.sub.2, from about 2.5 to about 7.5 mole %; and PA1 B.sub.2 O.sub.3, from about 86.5 to about 96.5 mole %; and PA1 said fibers being composed of a ceramic which PA1 Na.sub.2 O 1.5-4.5 mole % (.about.1.9 most preferred) PA1 SiO.sub.2 2.5-4.0 mole % (.about.2.9 most preferred) PA1 B.sub.2 O.sub.3 92-96 mole % (.about.95.2 most preferred). PA1 Na.sub.2 O 25-34 mole % PA1 B.sub.2 O.sub.3 50-66 mole % PA1 SiO.sub.2 0-11 mole % PA1 NaCl 0-6 mole % PA1 Na.sub.2 29.0-30.5 mole % (.about.29.8% most preferred) PA1 B.sub.2 O.sub.3 58-60 mole % (.about.59.5% most preferred) PA1 SiO.sub.2 5.5-6.5 mole % (.about.5.9% most preferred) PA1 NaCl 4.5-5.5 mole % (.about.4.8% most preferred).
A number of reasonably long-lived sodium sulfur cells have been made with the foregoing materials. However, cells of this type often fail prematurely and the apparent cause of failure frequently is breakage of some of the fibers where they emerge from the tubesheet. It is believed that the fibers are embrittled and/or weakened by a fluxing action of the molten tubesheet material during the fusion step. Of course, weakening of the fibers in the foregoing manner also greatly increases the susceptibility of the fibers to breakage during cell assembly.
When an alternative, otherwise suitable, tubesheet material (Na.sub.2 O, 18.80 B.sub.2 O.sub.3) was employed, similar results were experienced, i.e., the fluxing problem was not solved.
A need for a tubesheet material which meets all of the other requirements listed above, but which does not so severely flux the fibers, is apparent.
The sodium borate tubesheet glasses previously used are soft and quite difficult to adequately grind. The use of special grinding agents and techniques (see the cited U.S. Pat. No. 3,197,490) is required. An easier-to-grind tubesheet material is also highly desirable.
Sodium borosilicate glasses of widely varying proportions of sodium, silicon and boron oxides are well known. However, such glasses are not obvious candidate tubesheet materials. The addition of SiO.sub.2 to the sodium borate tubesheet materials might improve their grindability but also would tend to make them more nearly resemble the fiber composition --thereby perhaps increasing, rather than decreasing, the tendency for fluxing to occur.